IGNEOUS ROCKS/Granite 239 magmas is elevated by the carriage of unmelted source material (restite) Furthermore, it is becoming increasingly apparent from geophysical imaging (seismic, gravity), analysis of magnetic directional fabrics, and detailed field mapping that the three-dimensional geometries of many granitic plutons are tabular or funnel-shaped, with inclined feeder zones, and that plutons have an internally sheeted structure on a metre to kilometre scale The implication is that, rather than being intruded in a single episode, granitic plutons (and, on a larger scale, batholiths) were constructed sequentially and in situ by the coalescence of multiple sill-like magma pulses The most compelling demonstrations of this are in the Himalayas and Greenland Caledonides (see Europe: Scandinavian Caledonides (with Greenland)), where great topographical relief and superb exposure reveal that leucogranitic bodies have laccolithic geometry, where the constituent lenticular intrusions are separated by thin screens of country rock The same situation applies in some magmatic arcs Gravity surveys show that the vast Coastal Batholith of Peru is floored at surprisingly shallow depths (less than km) and comprises a collage of numerous sheet-like plutons Tabular plutons in the upper crust are fed by an underlying magma conduit, and, as highlighted above, this can occur in several ways Networks of feeder dykes are clearly exposed beneath the sheeted leucogranites of the Himalayas The importance of felsic-magma transport by dyking in the brittle uppermost crust receives further support from the geometry of sub-volcanic ring complexes, in which magma emplacement occurs as ‘ring dykes’ in conical fractures Some of the best examples of these are the alkaline ring complexes of the British Tertiary Igneous Province One important consequence of incremental and dynamic models of pluton assembly is that the ‘room’ problem is largely circumvented The incoming granitic magmas are progressively accommodated by a combination of tectonism – involving movement on pre-existing tensional faults and fractures and ponding of magma in dilatant jogs or pull-apart structures – and roof lifting during pluton inflation or ballooning, with subsidence of the floor of the magma chamber, possibly in response to magma extraction at depth The proximity of many plutons to major fault systems and their parallelism to regional strain patterns, as is especially evident in the Circum-Pacific subduction-related batholiths, support the notion of structurally controlled emplacement However, it is worth noting that plutons emplaced late in the orogenic history overprint deformational fabrics and are randomly aligned relative to the structural grain There is sometimes evidence for the localized stoping and assimilation of the roof rocks (Figure 4B), though this is probably a late-stage phenomenon rather than a major ascent mechanism Enclaves Pieces of rock enclosed by granitic bodies that exhibit a colour or textural contrast with the surrounding granite are termed enclaves Although comprising a small volumetric portion of most plutons, enclaves are eye-catching and exhibit a bewildering array of colours, shapes, sizes, textures, and compositions Enclaves are a source of information about the sorts of processes that may have operated during the evolution of granitic magmas, and so their interpretation is an important aspect of granitic studies The main enclave types encountered in granitic rocks are listed in Table Some are xenoliths spalled from the country rock, but the origin and significance of other enclave varieties remain debated High-grade metamorphic enclaves have been entrained from depth, though whether they are restite from the granitic source or simply mid-crustal xenoliths is difficult to determine (Figure 4C) A restitic origin seems reasonable for those enclaves with melt-depleted compositions, such as the cordierite–spinel lumps found in some peraluminous granites Mafic igneous enclaves have basaltic to dioritic compositions and are derived from the injection and fountaining of syn-plutonic mafic magma into the granitic pluton whilst it was partially crystalline (Figure 4D) As a result of the thermal and rheological contrasts between the hot fluid basaltic magma and the cool viscous granite, the mafic enclave commonly exhibits quenched margins and complex involute shapes resulting from contraction In appearance, mafic igneous enclaves are transitional to microgranular enclaves These may also have a finer-grained margin, though, rather than chilling, in some instances this is related to concentration of biotite resulting from a reaction between the iron- and magnesium-rich enclave and the potassium- and water-rich granite Microgranular enclaves have unusual igneous-looking textures, and their mineralogy and composition commonly, though not invariably, correlate with those of the host They are variously considered to represent an intermingled globule of relatively mafic magma that has undergone hybridization with the host granite, a reincorporated chilled margin derived from a less-evolved facies of the host, or partially melted restite entrained from the source Each of these origins may be correct in specific instances A mingling origin seems apposite where the enclave exhibits features that confirm its incorporation in the molten state, such as magmatic